Environmental Effects of Emissions

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Abstract: Air pollutants are responsible for a number of adverse environmental effects, such as photochemical smog, acid rain, death of forests, or reduced atmospheric visibility. Emissions of greenhouse gases from combustion of fossil fuels are associated with the warming of the Earth’s climate. Certain air pollutants, including black carbon, not only contribute to global warming, but are also suspected of having an immediate effect on regional climates.

Pollutants and Air Quality

Air pollutants are substances that adversely affect the environment by interfering with the physiology of plants, animal species, and entire ecosystems, with human property in the form of agricultural crops or man-made structures, and with climate. As global climate change has been considered among the key environmental challenges faced by humanity, certain climate forcing agents that otherwise cause no harm to living ecosystems—most importantly carbon dioxide—have been re-classified as air pollutants, along with such compounds as oxides of nitrogen or sulfur. On the other hand, climate research has linked certain compounds long recognized as air pollutants (for instance black carbon) to the warming of climate, thus providing one more reason for their control.

Air pollutants can originate from natural or anthropogenic (man-made) sources, or both. Examples of natural sources of pollution include volcanic eruptions or wind erosion. Emissions from internal combustion engines are an exemplary source of anthropogenic pollution. Some sources of pollution, such as forest fires, can be related to both natural phenomena and human activities.

Atmospheric reactions can transform primary pollutants into different chemical species. These reactions can produce both harmless compounds and secondary air pollutants that may be more harmful than their precursors.

The most important air pollutants, their sources, and known or suspected environmental effects are listed in Table 1 (after [298]).

**Table 1**
Air pollutants, their sources, and effects
Pollutant	Natural Source	Anthropogenic Source	Environmental Effect
Nitrogen oxides (NO + NO₂)	Lightning, soil bacteria	High temperature fuel combustion—motor vehicles, industrial, and utility	Primary pollutants that produce photochemical smog, acid rain, and nitrate particulates. Destruction of stratospheric ozone. Human health impact.
Particulates	Forest fires, wind erosion, volcanic eruption	Combustion of biofuels such as wood, and fossil fuels such as coal or diesel	Reduced atmospheric visibility. Human health impact. Black carbon particulates contribute to global warming.
Sulfur dioxide	Volcanic eruptions and decay	Coal combustion, ore smelters, petroleum refineries, diesel engines burning high-sulfur fuels	Acid rain. Human health impact.
Ozone	Lightning, photochemical reactions in the troposphere	Secondary pollutant produced in photochemical smog	Damage to plants, crops, and man-made products. Human health impact.
Carbon monoxide	Negligible	Rich & stoichiometric combustion, mainly from motor vehicles	Human health impact
Carbon dioxide	Animal respiration, decay, release from oceans	Fossil fuel and wood combustion	Most common greenhouse gas
Non-methane hydrocarbons (VOC)	Biological processes	Incomplete combustion, solvent utilization	Primary pollutants that produce photochemical smog
Methane	Anaerobic decay, cud-chewing animals, oil wells	Natural gas leaks and combustion	Greenhouse gas
Chlorofluorocarbons (CFC)	None	Solvents, aerosol propellants, refrigerants	Destruction of stratospheric ozone

Criteria Air Pollutants. Governments and international organizations have been taking action to protect the quality of air, as well as—in more recent years—to control emissions of climate forcing agents. Ambient air quality standards and guidelines, issued by environmental protection authorities, are instrumental in achieving air quality objectives.

An example of such legislation is the US National Ambient Air Quality Standards (NAAQS) program by the Environmental Protection Agency (EPA). The NAAQS address both human health (primary standard) and public welfare (secondary standard) concerns. Primary standards protect sensitive members of the human population from adverse health effects of criteria air pollutants. Secondary standards protect the public welfare from any known or anticipated adverse effects associated with the presence of a pollutant in the ambient air. Welfare effects include effects on soils, water, crops, vegetation, manmade materials, animals, wildlife, weather, visibility, climate, damage to and deterioration of property, hazards to transportation, as well as effects on economic values and personal comfort and well-being.

Under the US Clean Air Act of 1990, the NAAQS standards set maximum ambient concentration limits for six criteria pollutants including:

In 1997, standards were adopted for PM_2.5, or particulates below 2.5 µm, in addition to the PM₁₀ category. Urban areas are required to achieve attainment in regard to ambient concentrations of these criteria pollutants. Selected US, EU, and WHO air quality standards and guidelines were shown under Exposure to Engine Emissions.